1. Field of the Invention
This invention relates to drill bits having an annular pressure exterior of the drill bit when the drill bit is rotated in the borehole. The present invention relates more particularly to a drill bit for providing in a flowing fluid a negative pressure differential between a cutting area of the drill bit and a pressure reference location, such as within the annulus of the borehole exterior to the drill bit.
2. Setting of the Invention
During a drilling process, a drill bit has a drilling efficiency, measured as the rate of penetration through the rock. The drilling efficiency decreases significantly as the differential pressure between the borehole pressure and the pore pressure of the rock being drilled increases. One of the reasons for this phenomenon is caused by the high borehole pressure and subsequent high bit face pressure resisting rapid movement of the cut material in front of the cutting members of the drill bit. This phenomenon can be at least partially offset or improved by decreasing the pressure immediately in front of a cutting member of the drill bit. This can be done by locally accelerating the drilling fluid through a channel passing in front of the cutting member.
It is believed that such acceleration would occur in prior drill bits which conduct drilling fluid through slots which extend adjacent drill bit cutting members and which have cross-sectional areas smaller than the axial bores defined through the bits and connected to the slots. See U.S. Pat. No. 3,938,599 to Horn and U.S. Pat. No. 4,727,946 to Barr, et al. With the acceleration of the fluid, there would occur a drop in pressure in accordance with Bernoulli's principle. Although this pressure drop can favorably affect drilling efficiency, it has heretofore been limited to a nonnegative pressure differential relative to a reference pressure, such as the borehole pressure in the annulus (i.e., the annular pressure). As illustrated in FIG. 1 hereof and further explained hereinbelow, the pressure in a narrow slot in front of a cutting member would be greater than the annular reference pressure. From FIG. 1, the pressure would be within the range between the annular pressure (P.sub.annulus) and the bit pressure at the top of the bit (P.sub.bit). The pressure differential, taking P.sub.annulus as the reference, would be within the range between zero (P.sub.annulus - P.sub.annulus) and P.sub.bit -P.sub.annulus.
A significant result of enhancing drilling efficiency is the potential for reducing drilling costs; therefore, it is desirable to seek new ways to improve drilling efficiency. One way would be to overcome the aforementioned limit of the pressure drop adjacent a cutting member relative to a reference pressure in the annulus. Thus, there is the need for a drill bit constructed so that the pressure adjacent a cutting member can be dropped even below annular pressure whereby the pressure differential (.DELTA.P) between the cutting member and the annulus would not be limited to the range of 0.ltoreq..DELTA.P.ltoreq.(P.sub.bit -P.sub.annulus). That is, there is the need for a drill bit constructed so that a negative pressure differential between a cutting member and the annulus can be obtained whereby the absolute pressure at the cutting member is less than the annular pressure.